Filename: 317-secure-dns-name-resolution.txt
Title: Improve security aspects of DNS name resolution
Author: Christian Hofer
Created: 21-Mar-2020
Status: Needs-Revision

Overview:

   This document proposes a solution for handling DNS name resolution within
   Tor in a secure manner. In order to achieve this the responsibility for
   name resolution is moved from the exit relays to the clients. Therefore a
   security aware DNS resolver is required that is able to operate using Tor.

   The advantages are:

   * Users have no longer to trust exit relays but can choose trusted
     nameservers.
   * DNS requests are kept confidential from exit relays in case the
     nameservers are running behind onion services.
   * The authenticity and integrity of DNS records is verified by means of
     DNSSEC.

Motivation:

   The way how Tor resolves DNS names has always been a hot topic within
   the Tor community and it seems that the discussion is not over yet.

   One example is this recent blog posting that addresses the importance of
   avoiding public DNS resolvers in order to mitigate analysis attacks.

   https://blog.torproject.org/new-low-cost-traffic-analysis-attacks-mitigations

   Then there is the paper "The Effect of DNS on Tor’s Anonymity" that
   discusses how to use DNS traffic for correlation attacks and what
   countermeasures should be taken. Based on this, there is this interesting
   medium article evaluating the situation two years after it was published.

   https://medium.com/@nusenu/who-controls-tors-dns-traffic-a74a7632e8ca

   Furthermore, there was already a proposal to improve the way how DNS
   resolution is done within Tor. Unfortunately, it seems that it has been
   abandoned, so this proposal picked up the presented ideas.

   https://gitweb.torproject.org/torspec.git/tree/proposals/219-expanded-dns.txt

Design:

   The key aspect is the introduction of a DNS resolver module on the client
   side. It has to comply with the well known DNS standards as described in a
   series of RFCs. Additional requirements are the ability to communicate
   through the Tor network for ensuring confidentiality and the implementation
   of DNS security extensions (DNSSEC) for verifying the authenticity and
   integrity. Furthermore it has to cover two distinct scenarios, which are
   described in subsequent sections.

   The resolution scenario, the most common scenario for a DNS resolvers, is
   applicable for connections handled by the SocksPort. After successful socks
   handshakes the target address is resolved before attaching the connection.

   The proxy scenario is a more unusual use case, however it is required for
   connections handled by the DNSPort. In this case requests are forwarded as
   they are received without employing any resolution or verification means.

   In both scenarios the most noticeable change in terms of interactions
   between the resolver and the rest of Tor concerns the entry and exit points
   for passing connections forth and back. Additionally, the entry_connection
   needs to be extended so that it is capable of holding related state
   information.

Security implications:

   This improves the security aspects of DNS name resolution by reducing the
   significance of exit relays. In particular:

   * Operating nameservers behind onion services allows end-to-end encryption
     for DNS lookups.
   * Employing DNSSEC verification prevents tampering with DNS records.
   * Configuring trusted nameservers on the client side reduces the number of
     entities that must be trusted.

Specification:

   DNS resolver general implementation:

      The security aware DNS resolver module has to comply with existing DNS
      and DNSSEC specifications. A list of related RFCs:

      RFC883, RFC973, RFC1035, RFC1183, RFC1876, RFC1996, RFC2065, RFC2136,
      RFC2230, RFC2308, RFC2535, RFC2536, RFC2539, RFC2782, RFC2845, RFC2874,
      RFC2930, RFC3110, RFC3123, RFC3403, RFC3425, RFC3596, RFC3658, RFC3755,
      RFC3757, RFC3986, RFC4025, RFC4033, RFC4034, RFC4035, RFC4255, RFC4398,
      RFC4431,RFC4509, RFC4635, RFC4701, RFC5011, RFC5155, RFC5702, RFC5933,
      RFC6605, RFC6672, RFC6698, RFC6725, RFC6840, RFC6844, RFC6891, RFC7129,
      RFC7344, RFC7505, RFC7553, RFC7929, RFC8005, RFC8078, RFC8080, RFC8162.

   DNS resolver configuration settings:

      DNSResolver: If True use DNS resolver module for name resolution,
        otherwise Tor's behavior should be unchanged.

      DNSResolverIPv4: If True names should be resolved to IPv4 addresses.

      DNSResolverIPv6: If True names should be resolved to IPv6 addresses. In
        case IPv4 and IPv6 are enabled prefer IPv6 and use IPv4 as fallback.

      DNSResolverRandomizeCase: If True apply 0x20 hack to DNS names for
        outgoing requests.

      DNSResolverNameservers: A list of comma separated nameservers, can be an
        IPv4, an IPv6, or an onion address. Should allow means to configure the
        port and supported zones.

      DNSResolverHiddenServiceZones: A list of comma separated hidden service
        zones.

      DNSResolverDNSSECMode: Should support at least four modes.
        Off: No validation is done. The DO bit is not set in the header of
             outgoing requests.
        Trust: Trust validation of DNS recursor. The CD and DO bits are not set
               in the header of outgoing requests.
        Porcess: Employ DNSSEC validation but ignore the result.
        Validate: Employ DNSSEC validation and reject insecure data.

      DNSResolverTrustAnchors: A list of comma separated trust anchors in DS
        record format. https://www.iana.org/dnssec/files

      DNSResolverMaxCacheEntries: Specifies the maximum number of cache
        entries.

      DNSResolverMaxCacheTTL: Specifies the maximum age of cache entries in
        seconds.

   DNS resolver state (dns_lookup_st.h):

      action: Defines the active action. Available actions are: forward,
        resolve, validate.

      qname: Specifies the name that should be resolved or forwarded.

      qtype: Specifies the type that should be resolved or forwarded.

      start_time: Holds the initiation time.

      nameserver: Specifies the chosen nameserver.

      validation: Holds the DNSSEC validation state only applicable for the
        validate action.

      server_request: The original DNSPort request required for delivering
        responses in the proxy scenario.

      ap_conn: The original SocksPort entry_connection required for delivering
        responses in the resolution scenario.

   SocksPort related changes (resolution scenario):

      The entry point is directly after a successful socks handshake in
      connection_ap_handshake_process_socks (connetion_edge.c). Based on the
      target address type the entry_connection is either passed to the DNS
      resolver (hostname) or handled as usual (IPv4, IPv6, onion).

      In the former case the DNS resolver creates a new DNS lookup connection
      and attaches it instead of the given entry_connection. This connection is
      responsible for resolving the hostname of the entry_connection and
      verifying the response.

      Once the result is verified and the hostname is resolved, the DNS
      resolver replaces the target address in the entry_connection with the
      resolved address and attaches it. From this point on the entry_connection
      is processed as usual.

   DNSPort related changes (proxy scenario):

      The entry point is in evdns_server_callback (dnsserv.c). Instead of
      creating a dummy connection the received server_request is passed to the
      DNS resolver. It creates a DNS lookup connection with the action type
      forward and applies the name and type from the server_request. When the
      DNS resolver receives the answer from the nameserver it resolvers the
      server_request by adding all received resource records.

Compatibility:

   Compatibility issues are not expected since there are no changes to the Tor
   protocol. The significant part takes place on the client side before
   attaching connections.

Implementation:

   A complete implementation of this proposal can be found here:
    https://github.com/torproject/tor/pull/1869

   The following steps should suffice to test the implementation:

      * check out the branch
      * build Tor as usual
      * enable the DNS resolver module by adding `DNSResolver 1` to torrc

   Useful services for verifying DNSSEC validation:

   * http://www.dnssec-or-not.com/
   * https://enabled.dnssec.hkirc.hk/
   * https://www.cloudflare.com/ssl/encrypted-sni/

   Dig is useful for testing the DNSPort related changes:

      dig -p9053 torproject.org

Performance and scalability:

   Since there are no direct changes to the protocol and this is an alternative
   approach for an already existing requirement, there are no performance
   issues expected. Additionally, the encoding and decoding of DNS message
   handling as well as the verification takes place on the client side.

   In terms of scalability the availability of nameservers might be one of the
   key concerns. However, this is the same issue as for nameservers on the
   clearweb. If it turns out that it is not feasible to operate nameservers as
   onion service in a performant manner it is always possible to fallback to
   clearweb nameservers by changing a configuration setting.

